Photoresist is used for the production of circuit patterns on a printed circuit boards, of printing matrices for the printing industry, of semiconductor patterns on semiconductor devices, and of a variety of patterns used on various electronic devices.
A photoresist is a photosensitive material that is applied over a support or a substrate. A phototool or mask defining a pattern desired is applied over the photoresist. Exposing a negative-working photoresist through the mask to radiation such as ultraviolet light results in a pattern of hardened polymer where exposed (the effect of exposing positive-working photoresists would differ). The non-hardened photoresist in either case is then removed in a developing process leaving a pattern of hardened polymer. The areas of substrate not covered by the hardened polymer are then etched or coated with metal depending on the end product desired. The hardened polymer then is removed in a stripping process.
Materials used in this process for production of patterns on a substrate may include polymers to form photoresist mask or layer, photosensitive monomers or oligomers to act as a cross-linking agents, photo-polymerization initiators, stabilizers if necessary, and coloring agents. The polymers to form photoresist mask or layer may have acrylic acid ester, methacryl acid, itaconic acid and maleic acid as a copolymerizable component, and, further, carboxyl group as a side chain. The cross-linking agents may be monomers or oligomers which may have acryloyl groups or methacryloyl groups. The photo-polymerization initiator may be an anthraquinone analogue, benzophenone analogue, and benzoyl-ether analogue. The stabilizer may be hydroquinone. The coloring agent may be a dye stuff such as blue coloring matter and green coloring matter.
Waste solution discharged from the developing process and the stripping process of a printed-circuit board manufacturing process is an alkaline solution containing water-soluble photoresist material at the high concentration, and therefore, high chemical oxygen demand (COD), for example, 20,000 to 30,000 milligrams per liter (mg/l) and high Biological Oxygen Demand (BOD), for example, 1,000 to 4,000 mg/l. Before discharging such a waste solution to the environment, the solution must be treated to remove organics.
U.S. Pat. No. 4,786,417, incorporated herein by reference, teaches a process for removing the organics from the photoresist-containing solutions discharged respectively from developing and stripping and treating processes. Ultrafiltration membranes having specific characteristics are employed to concentrate and remove photoresist and other materials, allowing a clean permeate in each case to pass through the ultrafiltration membrane. The permeate in each case is pH adjusted before being recycled to the treating process from which it originated. Concentrated photoresist materials can be polymerized to form the solidified substance. Exposure to the electromagnetic radiation, such as sunlight, or/and subjection to heat energy can be used to accelerate polymerization of the photoresist material dissolved in the waste solution to thereby produce the solid of the polymerized (or cured) material.
Typical developing solutions may be aqueous solutions of 1 to 2 weight percent (wt %) sodium carbonate (Na.sub.2 CO.sub.3) containing 0.1 to 0.5 volume percent (vol %) of polyalkylene glycol as an antifoamer. They have a pH of about 10 to 11.5. Typical stripping solutions may be aqueous solutions of 1 to 5 wt % sodium hydroxide (NaOH) or potassium hydroxide (KOH), containing 0.1 to 0.5 vol % of polyalkylene glycol as an antifoamer plus various proprietary additives. They have a pH of about 13 to 13.5. As the developing solution or stripping solution is used, the Na.sub.2 CO.sub.3, NaOH or KOH, as the case may be, is depleted and photoresist material is dissolved. Periodically, the alkalizing agent must be replenished and waste solution containing the soluble photoresist materials must be purged.
U.S. Pat. No. 4,786,417 subjects the waste solution to ultrafiltration employing an ultrafiltration membrane having a cut-off molecular weight of 5,000 to 100,000. The patent indicates that the dissolved photoresist materials are densified or concentrated from about 1.5 wt % to a range of about 15 wt % to 30 wt % and the inorganic alkalizing agent is not concentrated any more, and the content thereof in the concentrated waste solution is as much as 1 to 2 %.
In general, membrane systems have been found to not adequately remove the color and to foul quickly, especially when exposed to non-aged waste solution. By non-aged waste solution, it is meant solution that is not exposed to sufficient sunlight or similar radiation after being removed from the developing or stripping solution. Typically, non-aged or fresh waste is that which exists when there are fewer than about eight hours between removal and filtration, but, if the waste is kept sealed in a container that prevents sunlight from contacting it, it behaves as fresh waste.